1. Field of the Invention
This invention relates to a process for manufacturing a double refraction plate used for various optical devices and more particularly to a novel process for manufacturing a double refraction plate according to which a transparent or light transmitting material such as metal oxide is deposited on a substrate from an oblique direction relative to the substrate surface.
2. Description of the Prior Art
A light arriving at a certain crystal or at an isotropic material placed in an electric field is refracted in two directions. This phenomenon is known as birefringence or double refraction, said crystal or the material having two discrete indices of refraction. This phenomenon is related with atomic orientation of the material or crystal and is caused by anisotropy proper of the material or crystal, that is, the characteristics of the physical properties thereof such as the rate of thermal expansion, electrical conductivity or thermal conductivity are varied with the direction in which the measurement of these properties is made. The light incident on the crystal plate showing these characteristics is split in two light wave components oscillating in two mutually perpendicular directions. In case the crystal plate is a uniaxial crystal, one of the light components is refracted in the usual manner of refraction through an isotropic medium. However, the refraction of the outer light component apparently does not obey the rule of refraction. Thus the index of refraction is varied with the direction of the plane of polarization. This difference in the index of refraction means that the speed of the light beam is varied with the direction in which it is proceeding.
Double refraction plates having the aforementioned characteristics are used in many optical components or devices making use of the anisotropic properties thereof with respect to light beams. Typical of them are wave plates or polarizers enclosed in an optical pickup for optical video disk units or an optical low-pass filter used in various image pickup devices. Above all, wave plates such as a quarter plate (1/4-wave plate) or an octant plate (1/8-wave plate) are formed by double refraction plates having the desired value of retardation and are used for modulating the plane of polarization as desired. By retardation is meant the optical path difference between two linearly polarized light components oscillating in mutually perpendicular directions.
Typical of the aforementioned crystals exhibiting double refraction are quartz and calcite. Thus the wave plates or polarizers are usually manufactured by cutting and polishing (grinding) the quarz for reducing their thickness. However, in this case, the polishing operation of the quartz or calcite must be performed with great accuracy, thus raising manufacturing costs. Moreover, because of limitation in size of the quartz or calcite, the resulting double refraction plate is also limited in size, thus making it impossible to manufacture a product of a larger size.
In place of the double refraction plate of quartz or the like, materials which are limited in size and expensive as mentioned above, it is also known to produce a double refraction plate by tensilizing a polymer film for providing an appropriate molecular orientation. With this process, the polymer film is tensilized in a certain direction for orienting and crystallizing the molecules of the polymer film in a specific direction for providing anisotropic properties of the crystal similar to those of quartz or calcite. In this manner, it has been possible to manufacture a double refraction plate having a larger size at lower costs. However, the double refraction plate with the desired retardation is extremely difficult to manufacture from the polymer film with improved accuracy.